Cryogenic vacuum pumps, or cryopumps, cooled by closed cycle refrigerators have gained wide acceptance in recent years for high vacuum applications. A cryopump typically includes a vacuum vessel, enclosing a cryopump chamber, which can be mounted to a work chamber which is to be evacuated. A cryogenic refrigerator is mounted to the vacuum vessel and a cold finger of the refrigerator extends into the vessel. The refrigerator typically operates on the Gifford-MacMahon cycle. High pressure refrigerant gas, such as helium, is introduced into the cold finger and there displaced and expanded by movement of a reciprocating displacer/regenerator. Cryopanels mounted to the cold finger within the cryopump chamber are cooled to cryogenic temperatures. Gases from the work chamber condense on those cryopanels and, with the gases thus removed from the work chamber, its pressure is reduced to a high vaccum in the order of 10.sup.-7 torr.
Because a cryopump is a capture system in which no moving parts are exposed to the vacuum environment, it is extremely clean. Cryopumps are also fast and efficient. Cryopumps cooled by closed cycle refrigerators are particularly convenient and economical because there is no need for an expendible cryogen such as liquid nitrogen. The refrigeration is developed in the cold finger which protrudes into the vacuum vessel. Cryopumps driven by closed cycle refrigerators have suffered one disadvantage which has made such cryopumps unsuitable for certain applications. In such applications as electron microscopes and electron beam lithography, vibrations generated in a closed cycle refrigerator can be detrimental.
In a conventional cryopump, the closed cycle refrigerator is mounted directly to the cryopump vacuum vessel. To minimize the vibration translated to the work chamber, systems have been proposed in which the refrigerator is vibration isolated from the work chamber. The vibration isolator may be positioned between the cryopump chamber and the work chamber or between the closed cycle refrigerator and the cryopump chamber. In order to provide for relative movement between the vibrating refrigerator and the work chamber, while still providing a vacuum seal between the refrigerator and the work chamber, a bellows is utilized. The bellows may surround the flow path between the cryopump chamber and the work chamber or it may surround the cold finger of the cryogenic refrigerator.
One form of isolator has been described by Guy S. Venuti in U.S. Pat. No. 4,363,217 and in "Use of Vibration Isolated Cryopumps to Improve Electron Microscopes and Electron Beam Lithography Units", Journal of Vacuum Science Technology, A1(2) April-June 1983, pages 237-240. The closed cycle refrigerator is suspended from the cryopump chamber by the bellows, and the bellows serve as an isolating spring. As a vacuum is created in the cryopump chamber, the difference in pressure, between the vacuum and ambient pressure, across the bellows tends to collapse the bellows. Collapse of the bellows is prevented by the suspended weight of the refrigerator and an additional mass mounted to the refrigerator. In such a system, the spring action of the bellows isolates the cryopump chamber from refrigerator vibration frequencies above a resonant frequency. Below the resonant frequency there is no isolation, and at the resonant frequency vibrations are actually amplified. By selection of the spring constant of the bellows and the mass suspended by the bellows, the isolator system can be designed to have a resonant frequency below the most significant vibration frequencies and thus effectively isolate the refrigerator from the work chamber. A primary deficiency of the Venuti system is that it requires the additional cumbersome mass. Further, that mass acts to stretch the bellows out when there is no vacuum in the vacuum chamber, so the refrigerator must have added support. Also, the system is only suited to cryopumps mounted below a work chamber.
In another form of vibration isolator which has been used with both cryopumps and turbomolecular pumps, the bellows is provided for vacuum sealing, and elastomer vibration isolators are positioned about the bellows between the refrigerator and the work chamber. Such a system relies on isolation and damping of vibration amplitudes by the elastomer isolators which also prevent collapse of the bellows. An example can be found in U.S. Pat. No. 4,539,822 to Sundquist. In a similar system, an elastomeric ring surrounds the bellows. A metal band around the elastomeric isolator ring prevents the collapse of the bellows and the cryopump vessel under vacuum. The elastomeric isolator ring provides both vibration isolation and damping of vibration amplitudes.